The proper orthogonal decomposition (POD) method is applied to analyze the pulsing spray characteristics of the fuel injection inside a four-valve optical spark-ignition direct-injection (SIDI) engine. The instantaneous spray structures are decomposed into four parts, namely the mean structure, large scale structure, transition structure and small scale structure, respectively, by using POD quadruple decomposition. The cycle-to-cycle variations (CCV) of the in-cylinder pulsing spray structure are examined separately based on the four parts. Analysis results indicate that the four parts have different characteristics, and each individual part represents a specific instantaneous spray structure. First, the mean part contains more than 90% of the total intensity of the spray field throughout the whole injection process. Moreover, the large scale structure part has the highest CCV level among all four parts, and it dominates the CCV of the entire spray field. The CCV of spray can be influenced by different engine operating conditions. In particular, the in-cylinder flow field has the strongest effect on the spray CCV. The varying motion of the in-cylinder flow field significantly influences the CCV of the large scale spray part, which in turn affects the CCV characteristics of the whole spray field. (C) 2015 Elsevier Ltd. All rights reserved.